We propose to test the hypothesis that a primary determinant for the rate of parathormone secretion from normal parathyroids is the intracellular turnover rate of the hormone. Given that hormone turnover has been demonstrated to occur in parathyroid cells, and that it is a physiologically meaningful process, we propose to identify the enzymes responsible for the process. The approach will be to identify first the primary cleavage products of parathormone cleavage such as the large, immunoreactive carboxy-terminal peptide which is secreted from normal parathyroid cells. Parathyroid cells will be incubated with radioactive amino acids to label certain residues within parathormone and its resultant hormone fragments. These fragments will be identified using the methods of affinity chromatography and more standard chromatographic methods for separation, followed by amino acid compositional and peptide sequencing techniques which will detect the levels and positions of the labeled amino acids. We will then isolate by standard methods those cellular proteases of the parathyroid such as cathepsins B and D which might cleave parathormone within the cell. The peptides produced by these enzymes in vitro will be compared to those found in situ in order to identify which enzymes are capable of reproducing the physiological cleavages. The results will be examined by inhibiting those enzymes in situ which are thought to function within the cell and to determine the effects on parathormone production and secretion. The results will be compared to those of experiments in which the formation and secretion of parathormone are modulated by extracellular factors such as calcium in order to determine how these agents influence intracellular hormone turnover. This type of comparison will also indicate whether other factors in addition to the rate of hormone turnover are important in terms of hormonal secretory rates.